Reinforced cylinder head gasket BMW N55B30

The BMW engines covered

The BMW N55B30 is the 3.0 litre single-turbo inline six-cylinder petrol engine fitted to the 135i, M135i, M235i, M2, 335i, 435i, 535i, 640i, 740i, X1, X3, X4, X5 and X6 in the compatibility table. Its bore is 84 mm with an 89.6 mm stroke, and all its versions receive the same original cylinder head gasket.

This original gasket carries the part number 11 12 7 599 212. BMW lists it as an asbestos-free cylinder head gasket without specifying its construction; its bore is 85 mm and its thickness 1.20 mm. The repair size, 11 12 7 599 213, measures 1.50 mm, which is 0.30 mm more. It is rated for standard combustion pressure: once that pressure climbs, clamping load fades, the cylinder head lifts and coolant passes into a cylinder.

Reinforced cylinder head gaskets

A head gasket does not make the seal on its own: the seal holds because the fasteners clamp the head against the block. A composite gasket stays crushed for good beyond 100 N/mm² of admissible stress, whereas the 301 stainless steel of MLX gaskets withstands 517 N/mm², 5 times more, and regains its shape at every combustion.

This page gathers the part numbers of Cometic, an American maker of racing gaskets, in a single construction: the MLX with its chamber stopper. The gasket bore is 85 mm, the same as the original gasket, and thicknesses run from 0.81 to 1.52 mm, enough to adjust the compression ratio without touching the pistons or the cylinder head. As a rule, the gasket bore chosen is at least 0.5 mm larger than the actual cylinder bore. Each part number is sold individually, one gasket per engine.

Technical specifications

Gasket volume: the free space the gasket leaves over each cylinder. It is added to the combustion chamber, so the bigger it is, the lower the compression ratio falls. Compare these values only between gaskets of the same bore.

Preparation and fitting

Before assembly, use a straight edge to check that the cylinder head and block are flat, and have them skimmed if necessary. These gaskets go on without sealant, as Cometic supplies them already coated.

Technical guide

The construction offered

Cometic MLX A multi-layer stainless steel gasket whose folded centre layer forms a stopper around the combustion chamber, with a high-temperature coating. Clamping rises locally without adding to bore distortion. It is the most resistant of the all-steel gaskets, to be chosen when boost becomes severe; it comes here in 85 mm only.

Choosing bore and thickness

Gasket bore. The Cometic MLX comes in 85 mm only.

Thickness. The original gasket is 1.20 mm thick; its repair size is 1.50 mm. A thicker gasket enlarges the combustion chamber and lowers the compression ratio; a thinner one reduces it and raises the ratio.

With a thickness below the original, use a dial gauge to measure how far the pistons protrude above the block: it sets the clearance that remains between piston and cylinder head.

The brand

Cometic Gasket, Inc., founded in 1989, is an American manufacturer of engine gaskets and sealing solutions based in Concord, Ohio. Its speciality is the gasket made to order, in the bore and thickness required. Its entire range is produced in the United States from American materials.

Advantages over the original gasket

When a reinforced gasket becomes necessary

• Adding forced induction, or raising boost on an already turbocharged engine
• Cylinder rebored beyond what the 85 mm bore of the original gasket covers
• Looking for a compression ratio different from standard, higher or lower
• Engine that has already lost its gasket, or is consuming coolant
• Competition use, where the gasket goes through repeated temperature rises

Vehicle compatibility

OEM part number: 11 12 7 599 212 in standard size, 11 12 7 599 213 in repair size.

Important: the N54B30, which the N55B30 replaced from 2010 on the 135i, 335i and X6 xDrive35i, receives another original cylinder head gasket, part number 11 12 7 557 265. Vehicles with the N54B30 engine are not covered by the part numbers on this page.

Why switch to a reinforced gasket?

When the original gasket reaches its limit, the reinforced gasket takes over: under higher combustion pressure it keeps its shape instead of being crushed.

When power climbs, through a turbo, a supercharger or a raised compression ratio, the head gasket turns into the first weak link of the top end. Choosing a reinforced gasket is then a matter of safety more than comfort: it avoids the failure and the head removal that follows it.